A high speed electrical load transfer system should satisfy a number of criteria in addition to being reliable and relatively inexpensive. It should minimize the variation in the power supply and should, in the event of power variation, such as gross failure, brown outs, harmonic distortion and the like, switch to an alternative power supply as soon as possible. The electrical load transfer system should minimize the amount and cost of components to increase reliability, and at the same time minimize cost of the system. It should also, in certain situations, provide capability for overload protection of a power supply wherein an inverter is used. Moreover, in an alternative power mode it should provide galvanic isolation upstream so that repair and maintenance of the primary power source can be performed in a safe manner.
U.S. Pat. No. 3,971,957 to HASE, issued Jul. 27, 1976 discloses an electrical load transfer control system which has two power sources, one being an inverter and the other being a commercial power source, where the load is critical and is normally connected to the inverter. The load transfer control system is provided to ensure an uninterrupted power supply to the load in the event of inverter failure. The load transfer control system includes a fault detector connected to the inverter, a fast acting and normally open switch connected to the commercial power source and to the load, and a slower acting and normally closed switch connected to the inverter and the load. In the apparatus, when an inverter fault is sensed, there is a momentary make-before-break switch action of the two switches. A ferroresonant circuit is in the inverter circuit, and has a storage element with enough energy storage capacity to carry the load during the period that it takes for the fault detector to operate and for the first switch to close. The switches are contactors or similar devices, so that there is complete voltage and current isolation of one power source from the other when the respective switch is open. The fault detector may operate in less than two milliseconds.
However, in many situations a response time of two milliseconds is far too long and critical loads such as computers, satellite links and the like must be switched much faster--in the order of one millisecond or less. In other situations, it may not be necessary to switch the critical load so fast, for example, in the case of brown outs. The above mentioned patent to HASE has certain disadvantages, namely that the response time is too slow for certain critical load switching, and also that it is inflexible in providing controlled switching in response to different failure conditions. Thus, the applicability of the load transfer control system of the aforementioned patent is somewhat limited; but on the other hand, it does permit no-break switching to be utilized.
An object of the present invention is to provide an improved load transfer system which obviates or mitigates one or more of the aforesaid disadvantages; and particularly when operating as part of a no-break UPS system.
This is achieved by providing a high speed load transfer system with a high speed transfer logic circuit which includes rapid sensing and rapid energizing means, so that in the event of a power failure, power to the critical load can be switched substantially instantaneously to an alternative power supply. A power supply detector in the form of a solid state chip which is responsive to a step input signal is used to substantially instantaneously sense a change in the primary power supply; and in the event of a gross failure, the output of the solid state chip is fed to a voltage amplification circuit which induces or ensures a rapid surge of current to actuate a power transfer switch, thereby to ensure substantially instantaneous connection of the critical load to an alternative power supply.
The power supply is normally a commercial AC line, with an alternative power supply in the form of a hot stand-by inverter. Alternatively, the power supply can be an inverter, with the alternative power supply in the form of a commercial AC line as a by-pass line.
The high speed load transfer system includes, in one embodiment, two discrete primary AC voltage sensing means. One is a step sensor as described above to sense rapid and large changes in input voltage, such as power failure; and the other is a differential sensor connected in parallel with said step sensor to sense failure due to other causes such as harmonic distortion and brown out, where the response of time of actuation is different from that for instantaneous power failure, and requires a different response. The differential and instantaneous primary voltage sensors operate in parallel to provide an efficient switch, and normally use a single common chip which minimizes the cost and redundancy of components.
The high speed load transfer system also includes a voltage amplification circuit in the form of a voltage doubler. The voltage doubler incorporates an AC input capacitor and a DC output capacitor coupled to a transfer contractor, which is usually a normally open/normally closed (NO/NC) contactor, but which may be a solid state switch for higher power applications. The DC capacitor is kept high to provide, on switching, a "high in-rush current" to speed up contactor operation and thereby to provide a very fast transfer. The AC input capacitor functions first as a doubler; and after the DC capacitor is discharged into the coil, the AC capacitor functions as an AC resistor with an impedance proportional to 1/ C, so as to supply and limit the DC voltage to the coil to a low DC voltage. As an example, for 140 volts in the voltage doubler circuit, there is a 300 volt output open circuit AC voltage, and a DC closed circuit voltage in the range of 10 to 48 volts--depending on the specific circuit components being used.
In one embodiment, the invention consists of a high speed load transfer system including an instantaneous step sensor plus a differential sensor, and further including the voltage doubler. That embodiment is particularly suitable for use with a hot stand-by inverter.
In another embodiment, the high speed load transfer system includes an instantaneous voltage step sensor together with a voltage doubler circuit and a current overload control circuit, and this is particularly suitable for use in a situation where power is supplied to a critical load from an inverter. In the event of failure of the inverter, power is switched to the critical load from a bypass line.
Thus, in one aspect of the invention there is provided a high speed load transfer system for use as a no-break load transfer system; whose purpose is for coupling an alternative source of AC voltage to a critical load in the event of interruption or failure of the primary AC voltage source.
The high speed load transfer system comprises a sensing means for detecting a change in the primary AC voltage source, where the sensing means has an output coupled to a switch means for switching the critical load between the primary AC voltage source and an alternative AC voltage source. Actuating means are associated with the switch means and are responsive to the output of the sensing means for actuating the switch means in the event of interruption or failure in the primary AC voltage source, so as to rapidly couple the critical load to the auxiliary AC voltage source.
In a second aspect of the invention, there is provided a high speed transfer logic circuit for use with a high speed load transfer system, to maintain an AC power supply to a critical load. This is essentially critical when the load transfer is to be effected within a no-break UPS system.
The high speed transfer logic circuit comprises a sensing means for sensing interruption or failure of an inverter AC power supply from an in-line inverter, where the sensing means has an output. Switch means are provided, which are responsive to an output from the sensing means to couple the critical load to an alternative AC power supply in the event of failure or interruption of the inverter AC power supply. The switch means has associated therewith means for providing a current surge through the switch means at the instant when the switch means closes, so as to minimize the time taken to couple the critical load to the alternative AC power supply.
Preferably the alternative AC power supply is a commercial AC bypass line.
Preferably also, the means for ensuring a current surge through the switch means is a voltage doubler circuit, consisting of a lower voltage AC input capacitor and a high voltage DC output capacitor. Conveniently, the voltage doubler circuit includes diode rectifying means coupled between the AC input capacitor and the DC output capacitor.
The DC output capacitor stores energy therein for fast transfers, after the switch means has closed to recharge the DC capacitor. Conveniently, the AC capacitor functions as a AC resistor after the switching has occurred, so as to limit the DC current to the switch means--which is typically a contactor coil.
Preferably, the high speed transfer logic circuit includes overload protection means for protecting the inverter in the event of overload of the inverter. The overload monitoring means comprises a current transformer means for monitoring the real output of the inverter, and for providing a DC voltage signal proportional to the inverter output. There is a voltage overload comparator means for comparing the current transformer output with a reference value, and timing means coupled to the overload comparator means. The timing means provides an overload output signal after a predetermined time, depending on the magnitude of the overload; and the overload output signal is coupled to switch means responsive to the overload signal, for coupling the critical load to the commercial AC bypass line. This gives a transfer logic circuit having Inverse Time Sensing; by which is meant that the higher the level of the overload, the faster will be the switching or response time to couple the critical load to the AC bypass line.
Ideally, the overload protection means includes interlock means coupled to the overload and transfer logic circuitry, for preventing re-transfer of the critical load until the overload rating falls beneath 100%. Conveniently, the interlock means includes manually resettable switch means, which resets the overload circuit so as to permit re-transfer of the load. It will be appreciated that alarm circuitry is included in the overload monitoring means.
It will also be appreciated that the voltage doubler circuit and overload circuitry can be applied to the inverter when used in a bypass or an in-line mode, and the voltage doubler circuitry is applicable to all load transfer devices to minimize the switching time and to ensure that the critical load is coupled in minimal time. The premise of a no-break UPS circuit is that there shall be no discontinuity of supply of power to a critical load, and that transfer of the load from the primary power source to an alternative power source shall be effected as quickly as possible. This may be especially important in the event of gross failure of the primary power source, or excessive overload when the primary power source is an inverter.
Accordingly, in other aspects of the invention there are provided methods of transferring a critical load at high speed to an alternative AC power source, and a method of rapidly energizing contactors using a circuit having a low voltage AC input capacitor and a high voltage DC output capacitor. In the methods according to this invention, rectifier means are coupled between the AC input capacitor and the DC output capacitor so that the output voltage across the DC capacitor is approximately double that at the input to the AC capacitor.
In another aspect of the present invention, there is provided a method of transferring a critical load to an alternative AC power source. The method comprises the steps of:
sensing substantially instantaneously a change in the AC supply to the critical load, by sensing reverse power flow from a tuned circuit;
generating an output signal in response to the change in supply, and feeding the output signal to a load transfer switch means; and
amplifying a voltage signal to be supplied to the load transfer switch means, so as to create a current surge to minimize the time taken to transfer the critical load.